In this research, fundamental experiments for application of photochromism to fluid measurements and flow visualization are carried out. Following results are obtained.(1) Absorbance of photochromic dyes; Absorbance, which depends on wave length of irradiated light, of 5 photochromic dyes of spiropyrans is investigated. The photochromic dyes are dissolved in ethanol. It is found that 1, 3, 3-trimethylindolino-6'-nitrobenzopyrylospiran has much larger absorbance in the ultra violet region than the other dyes. Therefore, this dye is very suitable for fluid measurements and flow visualization.(2) Length and density of dye trace For use of photochromism in fluid measurements and flow visualization, it is necessary that the most suitable density of dye and power of the laser are selected. The excimer laser beam (operated by XeCl, wave length = 308 nm) is used to form a dye trace. The dye is dissolved in ethanol. Length and thickness of the dye trace, depending on the density of dye (0.04-0.00075 weight %), are measured by a CCD camera. It is found that the length of the dye trace is longer for low density of dye than for high density. The optimum condition of density of dye is proposed for fluid measurements and flow visualization.